For example, I was thinking about broadcasting custom objects that can be airlifted using helos... I thought it would be fun to teamwork on slingload missions, although AFAIK there is no built-in support for visualizing slung loads incl. proper physics on other aircraft than your own - not 100% sure though...

In single-player mode I've been using the x-hoist plugin which provides an extension to X-Plane's built-in slung load feature including animated pick-up and drop-off locations. Similarly, I thought it would have been nice to be able to broadcast at least the pickup/dropoff locations of slung objects in JoinFS...

In general, X-Plane's plugin SDK allows for placing custom objects and modifying their location and angles each frame. Many plugins do that including the famous GroundTraffic addon whose source code is available on Github.